Geochemistry of the Cretaceous coals from Lamja Formation, Yola Sub-basin, Northern Benue Trough, NE Nigeria: Implications for paleoenvironment, paleoclimate and tectonic setting

Journal of African Earth Sciences 104 (2015) 56–70

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Geochemistry of the Cretaceous coals from Lamja Formation, Yola Sub-basin, Northern Benue Trough, NE Nigeria: Implications for paleoenvironment, paleoclimate and tectonic setting Babangida M. Sarki Yandoka a,b,⇑, Wan Hasiah Abdullah a, M.B. Abubakar b, Mohammed Hail Hakimi c, Adebanji Kayode Adegoke a,d a

Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia National Centre for Petroleum Research and Development, A.T.B.U, P.M.B 0248, Bauchi, Nigeria c Geology Department, Faculty of Applied Science, Taiz University, 6803 Taiz, Yemen d Department of Geology, Ekiti State University, P.M.B. 5363, Ado-Ekiti, Nigeria b

a r t i c l e

i n f o

Article history: Received 12 June 2014 Received in revised form 13 January 2015 Accepted 14 January 2015 Available online 28 January 2015 Keywords: Cretaceous coals Lamja Formation Paleoclimate Paleoenvironment Tectonic setting Yola Sub-basin

a b s t r a c t The Cretaceous coals of Lamja Formation located in Yola Sub-basin of the Northern Benue Trough, northeastern Nigeria, were analyzed based on a combined investigation of organic and inorganic geochemistry to define the paleodepositional environment condition, organic matter source inputs and their relation to paleoclimate and tectonic setting. The total organic carbon and sulfur contents of Lamja Formation coals ranges from 48.2%–67.8% wt.% and 0.42%–0.76% wt.%, respectively, pointing their deposition in freshwater environment with inferred marine influence during burial. Biomarkers and chemical compositions provide evidence for a major contribution of land-derived organic matter, with minor aquatic organic matter input. Minerals such as quartz, pyrite, kaolinite, illite, montmorillonite and calcite were present in the coals, suggesting that these minerals were sourced from terrigenous origin with slightly marine influence, considered as post-depositional. This is consistent with a significant amount of the oxides of major elements such as SiO2, Fe2O3, Al2O3, TiO2, CaO, and MgO. The investigated biomarkers are characterized by dominant odd carbon numbered n-alkanes (n-C23 to n-C33), moderately high Pr/Ph ratios (1.72–3.75), very high Tm/Ts ratios (18–29), and high concentrations of regular sterane C29, indicating oxic to relatively suboxic conditions, delta plain marine environment of deposition with prevalent contribution of land plants and minor aquatic organic matter input. Concentrations of trace elements such as Ba, Sr, Cr, Ni, V, Co and their standard ratios also suggested that the organic matter was deposited under oxic to relatively suboxic conditions, which is in parts deposited under marine influenced. Some standard binary plots of SiO2 versus (Al2O3 + K2O + Na2O) indicate a semi-arid paleoclimatic condition whereas log SiO2 versus (K2O/Na2O) also revealed passive continental margin setting. The inferred tectonic setting is in agreement with the tectonic events witnessed in the West and Central Africa during the Cretaceous period. Crown Copyright Ó 2015 Published by Elsevier Ltd. All rights reserved.

1. Introduction Coal is a multi-component of organic and inorganic materials. Organic matter is a major constituent in coal which determines the combustible energy and sources of hydrocarbons while, inorganic or mineral matter is a minor component, consisting of elements and minerals of environmental concerns that are hazardous during coal combustion in coal-fired power plants ⇑ Corresponding author at: Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia. Tel.: +60 166737410. E-mail address: [email protected] (B.M. Sarki Yandoka). http://dx.doi.org/10.1016/j.jafrearsci.2015.01.002 1464-343X/Crown Copyright Ó 2015 Published by Elsevier Ltd. All rights reserved.

(Romeo, 2014). The constituent of coal depends on the properties of country rocks, diagenesis, coalification process, depositional environments and mineralization as well as hydrological conditions (Sia and Abdullah, 2011; Arbuzov et al., 2011; Gürdal, 2011). Nigeria has the largest coal reserves in Africa with over 2 billion metric tons of which 650 million tons are proven (Adedosu, 2009; Obaje et al., 1999). Due to the importance of coal reserves in Benue Trough, the coals have been the subject of a large number of studies (e.g. Obaje et al., 1999; Akande et al., 2007; Jauro et al., 2007; Adedosu, 2009). Detailed organic and inorganic geochemical characterization of the Cretaceous coals from Lamja Formation in the Yola Sub-basin, are lacking. The integration of organic and

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inorganic geochemical approach was used to assess the paleodepositional environment and source of organic matter inputs in relation to paleoclimatic condition and tectonic setting. 2. Geological setting The Benue Trough is a major rifted basin in Nigeria (Fig. 1). It was formed from the tension generated, due to the separation of African and South American plates during the Early Cretaceous. Several authors have presented different tectonic models for the genesis of the Benue Trough (Abubakar, 2014; Sarki Yandoka et al., 2014). King (1950) proposed tensional movement resulting in a rift, while Stoneley (1966) proposed a graben-like structure. The RRF triple junction model leading to plate dilation and opening of the Gulf of Guinea was proposed by Grant (1971). Olade (1975) considered the Benue Trough as the third failed arm or aulacogen of a three armed rift system related to the development of hot spots. Benkhelil (1982, 1989), Guiraud and Maurin (1990, 1992) considered the wrench faulting as the dominant tectonic process during the Benue Trough evolution. The Benue Trough is geographically sub-divided into Southern, Central and Northern portions (Abubakar, 2014). The Northern Benue Trough is made up of N–W trending Gongola Sub-basin and the E–W trending Yola Sub-basin (Fig. 1). The stratigraphic succession of the Yola Sub-basin comprises the continental Early Cretaceous Bima Formation, the Cenomanian transitional marine Yolde Formation and the marine late Cenomanian–Santonian Dukul, Jessu, Sekuliye, Numanha Shales and Lamja Formations (Fig. 2). The Lamja Formation was described as ‘‘Carbonaceous Beds’’ by Carter et al. (1963). It consists of crystalline and shelly limestone, siltstone and yellowish to whitish fine-grained well bedded sandstone, dark grey shale and dark coals (Fig. 2) deposited in marine environment (e.g. Nwajide, 2013; Abubakar, 2006; Abubakar, 2014; Sarki Yandoka et al., 2015a).

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3. Sampling and methods Fourteen coal samples were collected from Lamja Formation (Fig. 2). The samples were collected using channel sampling after removing the weathering surfaces by digging to about 0.5 m. All samples were crushed to less than 200 meshes. About 0.50 g of each sample was used for the analysis of total sulfur (TS) and total organic carbon (TOC) contents using multi EA 2000 CS equipment. About 12 g of each sample was subjected to bitumen extraction with Soxhlet apparatus for 72 h using an azeotropic mixture of dichloromethane (DCM) and methanol (CH3OH) (93:7). The extracts were separated into saturates, aromatics and NSO (nitrogen, sulfur and oxygen) compounds by liquid column chromatography. The saturated hydrocarbon fractions were dissolved in hexane and analyzed by gas chromatography–mass spectrometry (GC–MS) on HP 5975B MSD mass spectrometer with gas chromatograph attached directly to the ion source (70 eV ionization voltage, 100 milliamps filament emission current, 230 °C interface temperature). X-ray Powder Diffraction (XRD) analysis was performed on the powdered sample using SIEMENS D5000 X-ray diffractometer with Cu Ka radiation, run from 5° to 60° 2h, with a step increment of 0.02° and a counting time of 2 s per step. The minerals were identified from the diffractograms by referencing to the ICDD Powder Diffraction File. About 0.50 g of each sample was prepared for non-destructive wavelength dispersive X-ray fluorescence spectrometer (PANalyticalAxiosmAX 4KW sequential XRF spectrometer). The XRF analysis was used to determine the concentration of oxides of major elements. About 0.50 g of each sample was weighed in a Teflon beaker and dried at 105 °C overnight. The dried samples were moistened with a few ml of deionized water. 5 ml of Nitric acid (HNO3) was slowly added and placed on hotplate at 150 °C, followed by 10 ml and 4 ml of hydrofluoric

Fig. 1. Regional tectonic map of western and central African rifted basins showing the Nigerian Benue Trough and study area (Adapted from United Reef Limited Report, 2004 and Abubakar, 2014).

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Fig. 2. Regional stratigraphic nomenclature of the Yola Sub-basin (modified after Jauro et al., 2007) and sedimentary log of Lamja Formation showing the location of the studied samples.

(HF) and perchloric acids (HClO4), respectively. The samples were digested with 10 ml of 5M HNO3 in a fume hood. The solutions were diluted with deionized water to 50 ml in a volumetric flask. All the digested samples were diluted up to 100 times with ultimate pure water (UPW). Standard solutions of the elements with

an analyte concentration of 10 ppm were used for calibration with a minimum detection limit of less than 1 ppb. The trace and rare earth elements were determined using Agilent Technologies 7500 Series Inductively-coupled plasma mass spectrometer (ICP-MS).

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4. Results and discussions 4.1. Molecular geochemistry 4.1.1. n-Alkanes and isoprenoids The chromatograms display full site of saturated hydrocarbons between n-C12–n-C33 n-alkanes and isoprenoid hydrocarbons (Fig. 3). The n-alkane distribution shows a bimodal distribution, with a predominance of high molecular weight compounds (nC23–n-C29). This support high terrigenous land-derived organic matter contribution with a minor aquatic organic matter input (Ebukanson and Kinghorn, 1986; Murray and Boreham, 1992). The distribution of n-alkanes is depleted in the n-C12–n-C19 range and show a dominance of the heavier members (n-C25+), which gave moderate to high CPI values ranging from 1.11 to 1.25

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(Table 1). Acyclic isoprenoids occur in a significant amount (Fig. 3). Pristane, generally occur in high relative concentrations in the analyzed coal extracts, possessing pristane/phytane (Pr/ Ph) ratios in the range of 1.75–3.75 (Table 1). This suggests that the coals were deposited under suboxic to relatively oxic conditions (Peters et al., 2005; Peters and Moldowan, 1993). Furthermore, higher amounts of isoprenoids pristane compared to nalkanes (Fig. 3) gave distinctively high pristane/n-C17 and low phytane/n-C18 ratios in the range of 2.14–5.83 and 0.82–1.25, respectively (Table 1). Waxiness index was calculated to provide some insights into the source input of the organic matter (Table 1). This index was used to determine the amount of land-derived organic matter in the sediments (Peters et al., 2005). The coals generally contain higher waxy ratios (>2.8; Table 1).

Fig. 3. Mass fragmentograms m/z 85 of saturated hydrocarbons of some studied coal samples.

60 Table 1 n-Alkane, isoprenoids and biomarker ratios of the Lamja Formation coals extracts calculated from m/z 85, m/z 191 and m/z 217 mass fragmentograms, respectively, illustrating source organic matter and depositional environment conditions.

LSS1A LSS1B LSS3A LSS3B LSS6A LSS6B LSS7A LSS7B LSS9A LSS9B LSS10A LSS10B LSS13A LSS13B

TOC (wt.%)

50.70 58.84 48.15 45.23 56.17 58.10 48.28 67.85 49.15 58.32 64.67 53.88 63.98 62.12

TS (wt.%)

0.69 0.76 0.74 0.75 0.45 0.68 0.73 0.69 0.67 0.64 0.49 0.42 0.69 0.66

Normal alkanes and isoprenoids

Triterpanes and terpanes (m/z 191) Hopane terpanes

Steranes and diasteranes (m/z 217) Tricyclic terpanes

Regular steranes (%)

Pr/ Ph

Pr/n C17

Ph/n C18

CPI

WI

C29N/ C30H

C30M/ C30H

Ol/ C30

Tm/ Ts

C26T/ C25T

C24Te/ C26T

C23T/ C24Te

C27

C28

C29

2.29 2.08 2.28 2.82 2.75 3.48 2.35 2.44 2.17 2.08 1.72 3.75 2.97 2.49

3.14 2.92 2.14 3.67 3.33 5.40 2.69 3.33 2.83 2.72 2.19 5.83 3.79 5.74

0.98 1.04 0.98 0.87 0.95 0.82 0.96 0.98 1.15 1.06 1.25 0.88 0.95 0.82

1.12 1.15 1.16 1.13 1.11 1.18 1.14 1.18 1.25 1.12 1.15 1.12 1.20 1.17

2.34 2.43 2.62 2.33 2.36 2.56 2.41 2.27 2.78 1.98 2.89 2.41 2.56 2.27

1.26 1.09 1.28 1.27 1.37 1.41 1.20 1.18 1.20 1.19 1.03 1.51 1.53 0.93

0.27 0.24 0.24 0.23 0.25 0.27 0.25 0.25 0.28 0.27 0.32 0.22 0.24 0.23

0.08 0.06 0.06 0.04 0.04 0.04 0.08 0.07 0.07 0.08 0.06 0.05 0.09 0.07

26 19 25 28 23 18 29 24 19 27 29 19 21 22

7.0 4.67 6.02 5.62 7.33 11.0 6.0 8.0 6.68 6.67 6.10 10 8.4 6.20

3.29 3.28 2.43 2.33 2.55 2.38 2.67 2.70 3.39 3.37 3.33 3.35 2.1 2.90

0.13 0.13 0.15 0.14 0.11 0.11 0.13 0.14 0.19 0.18 0.14 0.16 0.12 0.14

18.5 18.0 15.5 15.0 14.4 15.3 15.4 17.2 14.6 14.8 15.8 16.5 10.7 14.7

21.5 23.5 20.0 20.5 19.1 18.0 22.3 19.7 23.8 23.4 29.9 28.6 20.4 18.5

60.0 58.5 64.5 64.5 66.5 66.7 62.3 63.1 61.6 61.8 54.3 54.9 68.9 66.8

TS: Total sulfur (in weight percent). TOC: Total organic carbon (in weight percent). Pr: Pristane. Ph: Phytane. CPI: Carbon preference index: {2(C23 + C25 + C27 + C29)/(C22 + 2[C24 + C26 + C28] + C30)}. P P WI: Waxiness index – (n-C21–n-C31)/ (n-C15–n-C20). C29/C30: C29 norhopane/C30 hopane. C30M/C30H: C30 moretane/C30 hopane. Ol/C30H: Oleanane/C30 hopane. Ts: (C27 18a(H)-22,29,30-trisnorneohopane). Tm: (C27 17a(H)-22,29,30-trisnorhopane).

C29/ C27

C27/ C27 + C29

Diasterane/ sterane

Hopane/ sterane

3.24 3.25 4.16 4.30 4.62 4.36 4.05 3.67 4.22 4.18 3.44 3.33 6.44 4.54

0.24 0.23 0.24 0.19 0.26 0.18 0.19 0.21 0.22 0.19 0.23 0.24 0.13 0.18

0.54 0.55 0.56 0.46 0.53 0.58 0.52 0.49 0.50 0.58 0.59 0.60 0.51 0.39

36.2 37.9 36.2 35.2 38.3 33.7 36.3 40.3 38.6 40.9 35.6 33.8 39.4 40.2

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Sample ID

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4.1.2. Terpane and sterane biomarkers Terpane and sterane biomarkers were measured from m/z 191 and m/z 217 mass fragmentograms, respectively (Fig. 4a). Peak identifications of the m/z 191 and m/z 217 fragmentograms were made on the basis of their retention times and comparison of mass

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spectra with previously published works (e.g. Philp, 1985; Waples and Machihara, 1991; Abdullah, 1999; Alias et al., 2012). The identified peaks are listed in Appendices A and B. The m/z 191 mass fragmentograms of the saturated hydrocarbon fractions of all the analyzed coal extracts show moderate

Fig. 4. The m/z 191 mass fragmentograms (left) and m/z 217 mass fragmentograms (right) of saturated hydrocarbon fractions of some studied coal samples.

0.86 1.02 1.21 1.24 0.93 1.24 1.36 1.30 1.40 1.40 1.25 1.06 1.50 1.50 1.23 Al + K + N: (Al2O3 + K2O + Na2O). K/A: (K2O/Al2O3). K/N: (K2O/Na2O). Ti/Al: (TiO2/Al2O3).

Ni/Co V/Cr Sr/Ba

0.64 0.75 0.48 0.53 0.43 0.63 0.70 0.68 0.60 0.57 0.70 0.71 0.59 0.67 0.62 0.63 0.61 0.43 0.44 0.62 0.64 0.58 0.55 0.60 0.61 0.66 0.63 0.56 0.59 0.58

V/(V + Ni) V/Ni

1.67 1.53 0.75 0.78 1.61 1.78 1.36 1.22 1.52 1.54 1.93 1.71 1.27 1.42 1.44 0.45 0.43 0.46 0.65 0.40 0.51 0.64 0.46 0.54 0.49 0.44 0.42 0.41 0.54 0.49

Ga/Rb Ga

1.20 1.25 1.42 2.41 1.53 1.98 1.22 1.38 0.87 0.98 3.08 2.48 0.82 1.11 1.55 29.1 25.5 43.8 34.2 53.1 41.2 20.3 22.1 55.6 56.1 78.0 67.8 52.3 53.5 45.2

Ba Sr

18.5 19.2 21.2 18.1 22.8 26.1 14.3 15.1 33.1 32.2 54.8 48.1 30.6 35.7 27.8 2.75 2.85 3.06 3.66 3.75 3.85 1.89 2.98 1.59 2.01 6.94 5.96 1.85 2.02 3.23

Rb Cu

4.72 5.02 2.12 3.01 7.08 6.24 4.65 5.01 4.82 4.67 5.97 6.07 5.65 5.41 5.03 2.37 2.45 4.11 5.14 4.46 3.87 4.50 4.86 3.70 3.68 6.49 5.94 4.66 3.97 4.30

Ni Co

0.64 0.78 1.53 2.03 2.59 1.76 0.38 0.41 0.34 0.38 2.61 2.76 0.78 1.02 1.29 4.58 3.69 2.57 3.23 7.71 5.58 4.49 4.58 4.01 4.06 9.99 9.63 3.95 3.78 5.13

Cr V

3.96 3.76 3.12 4.02 7.20 6.92 6.12 5.95 5.62 5.67 12.5 10.2 5.92 5.67 6.19 300 300 3300 3400 2800 2800 100 200 200 200 2600 2700 200 300 1385.7 16.65 16.86 10.37 11.75 15.28 15.33 15.92 15.96 14.89 15.08 18.85 18.96 14.22 14.48 15.33 0.17 0.17 0.15 0.14 0.11 0.11 0.14 0.14 0.14 0.14 0.10 0.10 0.13 0.13 0.13 0.11 0.10 0.10 0.09 0.07 0.08 0.09 0.09 0.07 0.08 0.08 0.07 0.07 0.08 0.08 6.40 7.04 3.59 3.55 3.13 3.30 6.48 4.78 5.10 4.90 1.88 1.84 4.09 4.45 4.32 0.25 0.23 0.27 0.28 0.31 0.30 0.21 0.28 0.19 0.20 0.68 0.69 0.22 0.22 0.31 1.60 1.62 0.97 0.99 0.97 0.99 1.36 1.34 0.97 0.98 1.28 1.27 0.90 0.98 1.16 2.47 2.53 1.42 1.46 1.59 1.61 2.02 2.01 1.96 1.98 1.67 1.66 1.68 1.72 1.84 1.13 1.13 4.49 4.49 4.56 4.45 1.42 1.42 1.85 1.85 4.13 4.14 1.61 1.62 2.74 14.07 15.01 16.02 16.99 14.53 14.61 15.31 15.45 15.71 15.85 11.40 11.56 18.92 19.01 15.32 7.817 8.170 30.78 31.89 20.30 19.95 9.128 9.130 13.46 13.56 20.08 19.98 10.99 10.78 16.14

Al + K + N Ti/Al K/A K/N Na2O K2O TiO2 MgO Fe2O3 CaO Al2O3

14.82 15.02 9.46 10.78 13.99 14.03 14.37 14.40 13.72 13.89 16.89 17.01 13.21 13.45 13.93 36.16 38.87 26.38 25.11 39.26 39.01 39.96 39.98 37.15 37.78 39.34 38.98 35.24 36.03 36.38

Trace element compositions (in ppm)

MnO

Minor element compositions (wt.%)

SiO2

Most of the sulfur in coal is syngenetic, i.e. introduced during early diagenesis at the peat stage (Casagrande et al., 1980). The low sulfur content in freshwater coals, seem to originate from sulfur-containing amino acids and other organic compounds in the peat-forming plants, with very small contribution from pyrite (Casagrande et al., 1980; Chou, 2012). Total sulfur is considered as a more strong measure of the degree of marine influence than the organic or pyritic sulfur in sediments. Sykes (2004) adopted 0.5% sulfur content as the highest concentration of sulfur in the entire non-marine coals. As such, a total sulfur values >0.5% are taken to indicate some degree of marine influence, while values of 0.5–1.5% and >1.5% are classified as slightly and strongly marine influenced (Sykes et al., 2014). The total sulfur (TS wt.%) content of the coals ranges from 0.42% to 0.76% (Table 1). Thus, the samples were not concluded to have been formed under entirely freshwater conditions but, rather they have slightly marine influenced. This is consistent with the interpretation of the delta plain marine environment as supported by the presence of tricyclic biomarkers (Section 4.1.2). It is therefore suggested that, the marine influence was during the early burial from a downward percolation of sulfate rich water after coal measure formation. This is further supported by the presence of pyrite, mineral composition and elements as discussed below.

Table 2 Minor (wt.%) and trace element (ppm) compositions of the Lamja Formation coal samples.

4.2. Total sulfur content

LSS1A LSS1B LSS3A LSS3B LSS6A LSS6B LSS7A LSS7B LSS9A LSS9B LSS10A LSS10B LSS13A LSS13B Average

abundances of pentacyclic and tricyclic terpanes with higher amounts of tetracyclic terpanes (Fig. 4a). The relative abundance of C29 to C30 hopane is generally similar in most of the studied coal samples with C29/C30 ratios ranging from 0.8 to 1.0 (Table 1). The predominance of C29 hopane is frequently associated with carbonate-rich, but this is not always the case (Waples and Machihara, 1991), and the enhanced norhopane input may also be associated with land plant input (Rinna et al., 1996). The carbonate character of the coal samples is consistent with the mineral composition with CaO element in the range of 7.8–31.9% (Table 2). The Tm (C27 17a(H)-22,29,30-trisnorhopane) predominates over Ts (C27 18a(H)-22,29,30-trisnorneohopane) with Tm/Ts ratios ranging from 8 to 29 (Table 1). The 18a(H)-oleanane, which is an important land plant-derived biomarker (Peters and Moldowan, 1993; Peters et al., 2005) was identified in low proportion in almost all the analyzed samples (Fig. 4a). The studied coal samples display variable oleanane index (oleanane/C30 hopane) in the range of 0.04–0.09 (Table 1). Extended hopanes are dominated by the C31 homohopane and decreasing toward the C34 homohopane (Fig. 4a). The ab-hopanes are more prominent than the ba-hopanes while the S-isomers are more dominant than the R-isomers among the homohopane (C31–C34). The concentration of tricyclic terpanes is much less than that of tetracyclics in most of the coal samples (represented by C24 tetracyclic/C26 tricyclic; Table 1). The coals have relatively low C24/C23 and low C22/C21 tricyclic terpane values (Table 1), indicating significant terrigenous organic matter input but with minor marine-influenced depositional condition. The 17b,21a(H)-moretane was also detected in all the samples, though in low concentrations (Table 1). The steranes are another group of important biomarkers that are derived from sterols found in higher plants and algae, but rare or absent in prokaryotic organisms (Volkman, 1986). The m/z 217 mass fragmentograms of all the analyzed samples are dominated by steranes over diasteranes with C29 sterane being the predominant component (Fig. 4b). Relative abundances of C27, C28 and C29 regular steranes and the ratios of C29/C27 regular sterane, diasterane/sterane and hopane/sterane ratios are calculated and the results are given in Table 1. The Lamja Formation coals show a high proportion of C29 (54–69%) compared to C27 (11–19%) and C28 (18– 30%) steranes (Table 1).

3.70 3.14 2.69 2.53 1.72 2.20 11.84 11.85 10.88 9.68 2.49 2.15 5.97 3.89 5.34

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Sample ID

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4.3. Mineral compositions Most of the inorganic matter in coal is present as minerals dispersed throughout the coal macerals (Widodo et al., 2010). The dominant mineral of coals usually composed of sulfides, clay, carbonates and quartz (Stach et al., 1975). Sulfides are composed of pyrite and marcasite but pyrite is dominating by far and usually found associated with other sulfides or oxides in coal beds and as a replacement mineral in fossils (e.g. Mackowsky, 1943; Balme, 1956). In this study, the XRD analysis revealed that the Lamja Formation coals contain clay minerals, mainly kaolinite and illite, with a minor amount of montmorillonite (Fig. 5). Nonclay minerals such as quartz, feldspars, calcite and pyrite are also found (Fig. 5). The XRD results are generally compatible with the chemical analysis data and the mineralogical variation agreed with the minor element compositions (see Section 4.4). Higher amounts of illite indicate prevalence of more arid climate and occurrence of considerable amounts of kaolinite may suggest seasonal humidity (Chamley, 1989). Pyrite is present in the Lamja Formation coals (Fig. 6), suggesting that, the marine influence was during the early burial from a downward percolation of sulfate rich water after coal measure formation. The presence of pyrite in Lamja Formation coals is inferred to be precipitated post-depositional from descend-

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ing solutions percolating through sea-water strata which overlain the coal seam. This agrees with the marine depositional environment of the Lamja Formation. In addition, non-clay minerals such as quartz (Fig. 6) and feldspars are land-derived detritus whereas calcite is inherited from sea water. 4.4. Geochemistry of major elements Oxides of major elements (Table 2) in conjunction with mineralogical data were used to establish the element-mineral associations for Lamja Formation coals. SiO2, Fe2O3, CaO and Al2O3 are the dominant constituents with an average of 36.38 wt.%, 15.32 wt.%, 16.14 wt.%, and 13.93 wt.%, respectively (Table 2). Other major elements such as K2O, TiO2, MgO, Na2O, and MnO are also presence in low concentrations (Table 2). The elements Si, Al, Ti and K are mostly associated with quartz, clay minerals and feldspar as identified on XRD analysis (Fig. 5). These elements also revealed that Si, Al, Ti and K originate mostly from a mixed clay assemblage, which is consistent with the occurrence of kaolinite and illite identified (Fig. 5). SiO2 is the dominant constituent of the major elements in the Lamja Formation coals. This is consistent with the occurrence of quartz mineral identified based on XRD analysis (Fig. 5). The

Fig. 5. X-ray diffractograms of Lamja Formation coals showing the presence of Q-Quartz, Py-Pyrite, F-Feldspar, I-Illite, M-Montmorillonite, Cc-Calcium carbonate and KKaolinite.

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Fig. 6. Photomicrographs of Lamja Formation coal under oil immersion showing (a–c) pyrite (Py) crystals and, (d) quartz (Q).

contents of SiO2 may be probably caused by the interbedded sandstones and mudstones (e.g. Shu et al., 2013). Aluminum (Al) is an important component in the Lamja Formation coals (Table 2). The Al concentration in the coals is high compared with that of the world average, varying from 9.46 wt.% to 17.01 wt.% (Table 2). High Al in coal is related to the presence of clay minerals. High Al concentration in Lamja Formation coals is due to the presence of non-coal partings. The high Al concentration is explained by kaolinite and illite clay minerals observed by XRD analysis (Fig. 5). Aluminum (Al) is generally enriched in kaolinite (Hieronymus et al., 2001; Beckmann et al., 2005; Ratcliffe et al., 2004), while Potassium (K) is associated with illite clay mineral (Ratcliffe et al., 2004). The K/Al ratios in the Lamja Formation coals are relatively low (0.07–0.11), suggesting the occurrence of kaolinite higher than illite. Such enrichment of kaolinite was previously reported in much of the world’s coals (e.g. Vassilev and Vassileva, 1996). Titanium (Ti) originates mostly from clay assemblage, consistent with the occurrence of kaolinite and illite (Fig. 5). The higher Titanium (Ti)/Aluminum (Al) ratios are as a result of clay mineral input (Ross and Bustin, 2009). The Ti/Al ratios in the Lamja Formation coals are relatively high (0.10–0.17), suggesting the occurrence of Titanium (Ti) within clay lattices (Calvert et al., 1996; Ross and Bustin, 2009). Calcium (Ca) concentration in the Lamja Formation coals (Avg. 16 wt.%) is considerably higher than that of the world average of 1 wt.%. This suggests that, peat was deposited in mire supplied calcium-rich water (Stach et al., 1982). The high Ca concentration is explained by carbonate mineral observed in these samples based on XRD analysis (Fig. 5). Magnesium (Mg) in coal is related to clay minerals, particularly smectite and chlorite, as well as carbonates (Spears and Zheng, 1999). Magnesium concentration in Lamja Formation coals is 2.74 wt.%, much higher than the world average of 0.02 wt.% (Table 2). The higher Mg concentration is due to the enrichment of carbonates in Lamja Formation coals. This association is shown by good relationship with Ca (Fig. 7a).

Fig. 7. Relationship between minor elements of (a) calcium (Ca) and magnesium (Mg) and, (b) total sulfur content (TS) and iron element (Fe) for the investigated coal samples.

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Potassium (K) concentration in the Lamja Formation coals (Avg. 1.15 wt.%) is substantially higher than that of the world average (0.01 wt.%), (Sia and Abdullah, 2011). Higher K concentration is mainly attributed to the presence of illite clays in the analyzed coals (Fig. 5). Iron (Fe) in coal occurs in a number of forms, such as Fe sulfides (pyrite and marcasite), Fe carbonates (e.g., siderite and ankerite), Fe-bearing clays, Fe sulfates (e.g., ferrous sulfate and jarosite), and organically bound Fe (Wang et al., 2008). High Fe contents were recorded in the Lamja Formation coals with concentrations of 11.40–19.01 wt.%. Fe is correlated with S (r2 = +0.45; Fig. 7b), indicating that Fe mainly occurs in pyrite. XRD analysis confirmed that high Fe concentration corresponds with the presence of pyrites (Figs. 5 and 6). 4.5. Geochemistry of trace elements Trace element concentrations of the Lamja Formation coals are listed in Table 2 along with several widely geochemical ratios. Mn, Ba, Sr, V and Cr contents are prominent with average values of 1385.7, 45.2, 27.8, 6.19 and 5.13 ppm, while Cu, Ni, Rb, Ga and Co has average values of 5.03, 4.30, 3.23, 1.55 and 1.29 ppm (Table 2). Previous studies have demonstrated that, the concentrations of redox-sensitive elements, such as V, Ni, Cu, Cr and Mn, in sediments are sensitive indicators for paleo-redox conditions (e.g., Algeo and Maynard, 2004; Tribovillard et al., 2006; Fu et al., 2011; Mohialdeen and Raza, 2013). On the other hand, high concentration of trace elements such as Ba, Sr, Rb and V are indicative of seawater rather than freshwater (Reimann and de Caritat, 1998). Vanadium (V) and Nickel (Ni) are important indicators for redox conditions during deposition (Barwise, 1990; Galarraga et al., 2008). V is usually enriched in comparison with Ni in anoxic marine environments (Peters and Moldowan, 1993). V/Ni ratio greater than 3 indicates that sediments were deposited in a reducing environment, while V/Ni ratios ranging from 1.9 to 3 indicate deposition under suboxic conditions with precursor organic matter of mixed origin (Galarraga et al., 2008). V/Ni ratios for all the Lamja Formation coals ranges from 0.75 to 1.93 (Table 2), thus this show suboxic to relatively oxic conditions during deposition. More so, a high V/(V + Ni) value is an indication of anoxic bottom water conditions, while low V/(V + Ni) ratios usually reflect oxic to suboxic conditions (Lewan, 1984; Barwise, 1990). The relatively low V/(V + Ni) ratios (0.43–0.66) and sulfur contents (0.42–0.76 wt.%) denote that these coals were deposited under suboxic to oxic conditions. Strontium (Sr) and Barium (Ba) are two elements with different geochemical behavior. According to Liu (1980), the Sr/Ba ratio is regarded as an indicator of paleo-salinity. A high Sr/Ba ratio reflects high salinity, and a low Sr/Ba ratio indicates low salinity (Deng and Qian, 1993). The Lamja Formation coals have significant amounts of Sr and Ba ranging from 14.3–54.8 and 22.1–78.0, respectively. This may be related to the substitution of these elements for Ca due to similarity in charge and radius. The relatively high Sr/Ba ratio (avg. 0.62) (Table 2) indicates a moderately saline water during deposition. This is supported by the presence of limestone (Fig. 2), and thus has influence of marine conditions. Manganese (Mn) forms highly insoluble Mn3 + or Mn4 + hydroxides or oxides under oxic conditions, while during reducing conditions, is reduced to Mn2 + and forms soluble Mn2 + or MnCl + cations (Tribovillard et al., 2006), which can be dissolved within bottom sediments and diffuse into the overlying anoxic water column, where MnCO3 precipitation may occur, so that manganese concentrations in the seawater are depleted (Algeo and Maynard, 2004; Tribovillard et al., 2006). According to Quinby-Hunt and Wilde (1994), low manganese contents (average 75 ppm) are regarded as an indicator of highly reducing conditions.

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Therefore, Lamja Formation coals might have been deposited under suboxic to oxic conditions, as indicated by the low to high manganese concentrations (100–3400 ppm, see Table 2). Cobalt (Co) is also important indicator for paleo-redox conditions (Jones and Manning, 1994). Co is usually enriched in comparison with Ni in oxic conditions (Jones and Manning, 1994). Co concentrations in the coals are relatively low and range from 0.38 to 2.76 ppm (Table 2). According to Jones and Manning (1994), values of Ni/Co ratio below 5 indicate oxic environment whereas values of the same ratio above 5 suggest suboxic environment. The coals were considered to be deposited under suboxic to oxic conditions, as indicated by the Ni/Co ratio ranging from 1.72 to 11.85 (see Table 2). Chromium (Cr) concentration in the Lamja Formation coals (Avg. 5.13 ppm) is higher than that of the world low-rank coals. The modes of occurrence of Cr in coals are not very clear (Swaine, 1990) but Cr seems to be associated with organic matter (Dai et al., 2008; Finkelman, 1981; Mukherjee et al., 1988), and clay minerals (Rimmer, 1991). The inorganic association of Cr in coals is demonstrated by the negative relationship with total organic matter content (r2 = 0.40; see Fig. 8a). The apparent association of Cr with clays in coals can be deduced from the significant positive relationship between Cr and Al (r2 = +0.57; see Fig. 8b). Cr is usually enriched in comparison with V in oxic conditions and values of V/Cr ratio above 2 indicate anoxic condition whereas values of the same ratio below 2 suggest suboxic to oxic conditions. The coals have V/Cr ratio between 0.86 and 1.50, further suggesting suboxic to oxic conditions.

Fig. 8. Relationship of chromium (Cr) element versus TOC and aluminum (Al) contents of the studied coals.

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4.6. Paleodepositional environment conditions Organic geochemical (biomarker distributions) and chemical data (major and trace elements) were used to describe the source input of organic matter and depositional environment conditions of Lamja Formation coals. The n-alkane distributions are consistent with a dominant source of terrestrial higher plants although, receiving a minor organic matter input as indicated by a predominance of odd carbon number n-alkanes to even carbon number nalkanes (Fig. 3). The long chain n-alkanes (>n-C23) are characteristic biomarkers for higher terrestrial plants (Eglinton and Hamilton, 1967), whereas the short-chain n-alkanes (1.0) are obtained from the Lamja Formation coals (Table 1). These CPI values is accompanied by the presence of significant long-chain n-alkane compounds (+n-C23), thus supporting a terrigenous organic matter input deposited under relatively oxic conditions (Akinlua et al., 2007; Meyers and Snowdon, 1993). The pristane which is much higher than the phytane (Fig. 3) also supported this high terrigenous organic matter input (Peters et al., 2005). The pristane/phytane (Pr/Ph) ratio has been widely used as an indicator of the redox conditions in the depositional environment and source input of organic matter (Peters et al., 2005). Organic matter originating from terrestrial plants would be expected to contain high Pr/Ph ratio of >3.0 (oxidizing conditions), while low values of (Pr/Ph) ratio (<0.6) indicate anoxic conditions, and values between 1.0 and 3.0 suggest intermediate conditions (suboxic conditions) (Peters and Moldowan, 1993). The Pr/Ph ratios of the coals are in range of 1.72–3.75 (Table 1), thus show deposition under suboxic to relatively oxic conditions. Furthermore, pr/n-C17 and ph/n-C18 ratios suggest a significant contribution of terrigenous organic matter with small amount of aquatic organic matter input that were preserved under oxic to relatively suboxic conditions (Fig. 9). The relative distribution of C27, C28 and C29 regular steranes is graphically represented in the form of a ternary regular steranes diagram (Fig. 10 as adapted after Huang and Meinschein, 1979). The dominance of C27 steranes would indicate a preponderance of marine phytoplankton, whereas a dominance of C29 would indicate a strong land plant terrestrial contribution and C28 steranes might indicate a heavy contribution by lacustrine algae (Peters et al., 2005). Based on the ternary classification, the analyzed coals contain a high contribution of terrestrially derived organic matter with minor aquatic organic matter contributions (Fig. 10), which thus display a strong predominance of C29 steranes (Table 1). The C29/C27 regular sterane ratio range from 3.24 to 6.44 further supported the above interpretation (see Table 1). Oleanane is a strong indicator of terrestrial angiosperm plant as initially reported by Ekweozor and Telnaes (1990). The presence of oleanane suggests probable marine-influence as indicated by earlier work of Murray et al. (1997) and recently reported for Tertiary coals (Alias et al., 2012). The influence of marine condition is consistent with the sulfur content (>0.5 wt.%) and the presence of pyrite that were identified within the coals (Figs. 5 and 6). The depositional environmental conditions of the Lamja Formation coals have also been interpreted using geochemistry of major and trace elements. The presence SiO2 and TiO2 in significant amounts (Table 2) further confirms that the Lamja Formation coals are originated from terrestrial peats. Paleo-redox conditions during sedimentation of Lamja Formation coals can also be evaluated from trace elements data such as Mn, V, Cr, Ni and Co (Sageman and Lyons, 2004; Fu et al., 2011). Some standard ratios such as V/Ni, V/Cr, and Ni/Co are most commonly used as indicators of paleo-

Fig. 9. Phytane to n-C18 alkane (Ph/n-C18) versus pristane to n-C17 alkane (Pr/n-C17), showing depositional conditions and type of organic matter of the analyzed coal samples.

Fig. 10. Ternary diagram of regular steranes (C27–C29) showing the relationship between sterane compositions and organic matter input of the analyzed coal extracts (modified after Huang and Meinschein, 1979),

redox conditions (Table 2). The ratios of trace elements suggest that the Lamja Formation coals were deposited under suboxic to oxic conditions (see Section 4.5). In addition, the relatively high Sr/Ba and V/Ni ratios (Avg. 0.62 and 1.44, respectively) also reflect enhanced salinity stratification and suboxic to relatively oxic conditions.

4.7. Paleoclimatic conditions Paleoclimatic conditions during sedimentation of rocks can be evaluated from chemical analyses (e.g., Jacobson et al., 2003; Suttner and Dutta, 1986). However, paleoclimatic conditions can be inferred from major-, trace-, and rare earth-based discrimination diagrams proposed by several investigators (e.g. Suttner and Dutta, 1986; Hieronymus et al., 2001; Beckmann et al., 2005; Ratcliffe et al., 2004; Roy and Roser, 2013). In this study, the paleoclimatic conditions were evaluated based primarily on major and trace elements distributions. These include; SiO2, Al2O3, K2O, Na2O, Ga and Rb (Table 2). Suttner and Dutta (1986) proposed a binary SiO2 versus (Al2O3 + K2O + Na2O) diagram to restrict the

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paleoclimatic condition during sedimentation. Based on the plot of SiO2 versus (Al2O3 + K2O + Na2O), the Lamja Formation coals were formed during semi-arid climatic conditions (Fig. 10a). This is consistent with the moderate saline water during coal formation as indicated by relatively high Sr/Br ratios (Table 2). Aluminum (Al) and Gallium (Ga) are generally enriched in kaolinite and associated with warm and humid climate (Hieronymus et al., 2001; Beckmann et al., 2005; Ratcliffe et al., 2004), while Potassium (K) and Rubidium (Rb) are also associated with illite, reflecting weak chemical weathering and related to dry and cold climatic conditions (Ratcliffe et al., 2004). However, most sediment rich in illite should have low Ga/Rb and high K2O/Al2O3 ratios, whereas those that are rich in kaolinite will have high Ga/Rb and low K2O/Al2O3 ratios. The Lamja Formation coals have relatively high Ga/Rb and low K2O/Al2O3 ratios (Table 2), indicating that the kaolinite clay mineral is higher than illite clay mineral in the coals as identified by XRD analysis (Fig. 5). 4.8. Tectonic setting Several studies had been undertaken on the chemical compositions of sediments and it has shown that these sediments are significantly controlled by plate tectonic settings. Tectonic settings of provenances of ancient sediments can be inferred from major-, trace-, and rare earth-based discrimination diagrams proposed by several investigators (e.g. Maynard et al., 1982; Bhatia, 1983; Bhatia and Crook, 1986; Roser and Korsch, 1986). A number of classifications attempt to discriminate various origins and tectonic settings of sediments into three or four categories on the basis of bulk

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geochemical elements (e.g., oceanic island arc, continental island arc, active continental margin, passive margin (Maynard et al., 1982; Bhatia, 1983; Bhatia and Crook, 1986; Roser and Korsch, 1986). Roser and Korsch (1986) used log (K2O/Na2O) versus SiO2 (Fig. 11b) to determine the tectonic setting of sedimentary rocks. SiO2 and K2O/Na2O increase from volcanic-arc to active continental margin to passive margin settings. According to Roser and Korsch (1986), the Lamja Formation coals plot in the field of passive continental margin (Fig. 11b). This tectonic setting inferred for the provenance hosting the Lamja Formation coals is in agreement with the tectonic events witnessed in the West and Central Africa during Cretaceous period (Genik, 1993; Sarki Yandoka et al., 2015b). 5. Conclusions An integrated organic and inorganic geochemical investigation of the Lamja Formation coals in the Yola Sub-basin, to determine their paleodepositional environmental condition, organic matter source inputs in relation to paleoclimate and tectonic setting of the source region have revealed the following: (1) The coals were deposited in a delta plain marine environment under relatively freshwater and suboxic to relatively oxic conditions, as supported by biomarker environment characteristics and total sulfur content (>0.5%). (2) The saturated hydrocarbon fractions of the coals are characterized by dominant odd carbon numbered n-alkanes (n-C23 to n-C33) with moderate to high CPI values (1.11–1.25), high Pr/Ph ratios (1.72–3.75), high Tm/Ts ratios (18–29), high concentrations of regular sterane C29, as well as the presence of high abundance of tetracyclic terpanes, which is consistent with oxic to suboxic delta environment that de-signify a dominant contribution of land plants organic matter. Small amount of aquatic organic matter input is presence based on n-alkane and tricyclic biomarkers. (3) The dominant major elements identified in the coals are SiO2 followed by Fe2O3, CaO, Al2O3, and TiO2. The assemblages and modes of occurrence supported the terrigenous origin. Minor marine influence, considered as post-depositional is also evidenced based on the presence of pyrite associated with the organic matter. (4) Based on the assessment of the trace elements Sr, Ba, V, Ni, Co, Cr and their ratios, a stratified water column with moderate salinity and suboxic to relatively oxic bottom water conditions are evidenced within the coals and again, indicative of an influence of marine conditions. (5) The dominant clay minerals identified in the coals are kaolinite and illite as confirmed by geochemistry elements (e.g., Al, K, Ga and Rb). The assemblages and modes of occurrence of these elements indicate a semi-arid paleoclimatic conditions. (6) In addition, a standard binary plot of geochemistry elements such as SiO2 versus (K2O/Na2O), revealed passive continental margin setting for their provenance. This is consistent with the tectonic events witnessed in the West and Central Africa during the Cretaceous period.

Acknowledgements

Fig. 11. Bivariate plots of (a) SiO2 versus (Al2O3 + K2O + Na2O) contents for palaeoclimate discrimination (After Suttner and Dutta, 1986) and, (b) SiO2 versus K2O/Na2O tectonic setting discrimination diagram (after Roser and Korsch, 1986).

The authors wish to acknowledge the financial support by the National Centre for Petroleum Research and Development, Nigeria (NNBT Project) and University of Malaya, Malaysia (IPPP Research Grant Number: PG140-2012B). The authors are also grateful to Mr. Mohammed Zamri Rashid for analytical assistance.

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Appendix A Definitions and measurement procedures used in the literature.

22S/(22R + 22S) Ts/(Ts + Tm) C29-hop/C30-hop C22T/C21T C24T/C23T C26T/C25T C24Tet/C26T C30-mor/C30-hop 20S/(20S + 20R) bb/(bb + aa)

C3217a(H),21b(H)22S/[C3217a(H),21b(H)22(R + S)] 18a(H)-22,29,30-trisnorneohopane/[18a(H),22,29,30-trisnorneohopane + 17a(H),22,29,30-trisnorhopane] C2917a(H),21b(H)-hopane/C3017a(H),21b(H)-hopane C22 tricyclic terpane/C21 tricyclic terpane C24 tricyclic terpane/C23 tricyclic terpane C26 tricyclic terpane/C25 tricyclic terpane C24 tetracyclic terpane/C26 tricyclic terpane C3017b(H),21a(H)-hopane/C3017a(H),21b(H)-hopane C295a(H),14a(H),17a(H),20S/[C295a(H),14a(H),17a(H),20(S + R)] [5a(H),14b(H),17b(H)(20R + 20S)C29sterane]/ [5a(H),14b(H),17b(H)(20R + 20S) + 5a(H),14a(H),17a(H)(20R + 20S)]C29 steranes

Appendix B Peak assignments for saturated hydrocarbons fractions in the gas chromatograms (I) in the m/z 191 mass fragmentogram and (II) m/z 217 mass fragmentogram. Peak identity

Compound

Carbon no.

C19 C20 C21 C22 C23 C24 C24 C25 C26 18a(H),22,29,30-trisnorneohopane 17a(H),22,29,30-trisnorhopane 17a(H),29,30-bisnorhopane 17a(H)21b(H)-norhopane 18a(H),30-norneohopane 17b(H),21a(H)-hopane (normoretane) 17a(H),21b(H)-hopane 17b(H),21a(H)-hopane (moretane) 17a(H),21b(H)-homohopane (22S) (22S and 22R) 17a(H),21b(H)-homohopane (22S)

C19 C20 C21 C22 C23 C24 C24 C25 C26

19 20 21 22 23 24 24 25 26

C31ab

17a(H),21b(H)-homohopane (22R)

(I) Fragmentogram m/z 191

Ts Tm C28ab C29ab C29Ts C29ba C30ab C30ba C31ab C31ab 22S 22R C32ab (22S and 22R)

tricyclic (Cheilanthane) tricyclic (Cheilanthane) tricyclic (Cheilanthane) tricyclic (Cheilanthane) tricyclic (Cheilanthane) tricyclic (Cheilanthane) tetracyclic tricyclic (Cheilanthane) tricyclic (Cheilanthane) 27 27 28 29 29 29 30 30 31 31

17a(H),21b(H)-homohopane

31 32

C33ab

17a(H),21b(H)-homohopane

33

C34ab

17a(H),21b(H)-homohopane

34

C35ab

17a(H),21b(H)-homohopane

35

(22S and 22R) (22S and 22R) (22S and 22R) (II) Fragmentogram m/z 217

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Peak identity

Compound

Carbon no.

C27ba 20S C27ba 20R C27ab 20S C27ab 20R C27aaa 20S

13b, 17a(H)-diacholestane (20S) (diasterane) 13b, 17a(H)-diacholestane (20R) (diasterane) 13a, 17b(H)-diacholestane (20S) (diasterane) 13a, 17b(H)-diacholestane (20R) (diasterane) 5a(H),14a(H),17a(H)-cholestane (20S) (sterane) 5a(H),14a(H),17a(H)-cholestane (20R) (sterane) 24-methyl-5a(H),14a(H),17a(H)-cholestane (20S) (sterane) 24-methyl-5a(H),14b(H),17b(H)-cholestane (20R) (sterane) 24-methyl-5a(H),14b (H),17b (H)-cholestane (20S) (sterane) 24-methyl-5a(H),14a(H),17a(H)-cholestane (20R) (sterane) 24-ethyl-5a(H),14a(H),17a(H)-cholestane (20S) (sterane) 24-ethyl-5a(H),14b(H),17b(H)-cholestane (20R) (sterane) 24-ethyl-5a(H),14b(H),17b(H)-cholestane (20S) (sterane) 24-ethyl-5a(H),14a(H),17a(H)-cholestane (20R) (sterane) 24-ethyl-13b(H),17a(H)-diacholestane (20S) (diasterane) 24-ethyl-13b(H),17a(H)-diacholestane (20R) (diasterane)

27 27 27 27 27

C27aaa 20R C28aaa 20S C28abb 20R C28abb 20S C28aaa 20R C29aaa 20S C29abb 20R C29abb 20S C29aaa 20R C29ba 20S C29ba 20R C29ab 20S C29ab 20R

24-ethyl-13a(H),17b(H)-diacholestane (20S) (diasterane) 24-ethyl-13a(H),17b(H)-diacholestane (20R) (diasterane)

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